This invention relates, in general, to metallurgical fabrication techniques associated with aluminum alloys and in particular to a method for fabricating an improved structure of aluminum ladder. As such, it is an object of the invention to enable high strength ladder rungs to be easily manufactured by a process in which the high yield strength requirements of the central portion of the rung are retained during a selective heat treatment and formation process applied solely to the outer ends of the ladder rungs. The invention entails applying a selective heat treatment process to the outer portion of the ladder rungs only, which enables the ends of the ladder rungs to be first crimped, positioned within the end rails and subsequently flanged, without cracks or fractures occurring during or after the formation process. Localized heating of the ladder end rungs, in the preferred embodiment, is accomplished through the use of high frequency induction heating. And while high frequency induction heat treatment has been utilized in the past for annealing cold worked structures, it has not been contemplated for utilization in an overaging and solution annealing process on fully age-hardened alloy aluminum structures such as the present ladder structures, with the particular problems inherent in their construction.
Certainly prior art processes exist in which selected areas of pipes or tubes are heated during the process of bending of the pipe or tube. The use of heat application for this specific purpose in the past has sometimes been to prevent damage to a coating on the surface of the pipe or tube. Alternatively, heat was applied to a pipe or tube in order to assist in the bending of a pipe. In neither case was heat applied to the tubing for the specific purpose of increasing the formability of age-hardened tubing, while maintaining superior yield strength in the central portion thereof--while rendering increased strength in the formed portion after aging.
Because of the lack of the means by which ductility of the ends of ladder rungs could be increased, while still maintaining high strength at the central portion of the rung, manufacturers of heavy duty aluminum ladders have been limited to a somewhat complex process utilizing several steps including extrusion, cropping, quenching, room temperature aging for twenty-four hours, cold drawing over a mandrel and artificial aging. Most previous attempts at obtaining similar results through the use of a simplified process, comprising only the steps of quenching and artifical aging, have for the most part, proven unsuccessful. In one prior art process, "double-aging" has been utilized in which the entire ladder rung extrusion is first solution annealed, water quenched, then partially aged and cold drawn. The extrusion is then aged at a second temperature, one higher than the first aging temperature. Such a process can be time consuming and expensive.
The innovative process which is the subject of the present invention is capable of producing substantial savings in terms of energy, labor and time, through use of selective heating of ladder rung ends. Moreover, it allows ladder designers of heavy duty ladders to keep the rungs at the preferred weight per foot of 0.300 to 0.350 pounds per linear foot of rung width. And, as a result of these selective heat treatments, each rung so produced is further able to pass rigorous deflection tests in the industry in which a 1,000 pound test load is applied with a maximum permanent deflection of 1/25 of the rung span--all of which is required to obtain a "Type 1A" or "extra heavy duty" rating for a particular ladder design.
As noted above, although a heavy duty ladder rung must withstand a deflection of less than 1/25 of the rung span, upon being subjected to a 1,000 pound bending load for one minute, the metal additionally is expected to be capable of being completely folded back upon itself, while a collar is formed during the operation of securing the ladder rung to the end rails. Without adequate ductility the metal can fracture during the upset operation. As a result, many ladder manufacturers have been unable to consistently produce heavy duty ladders that are able to pass the severe formability requirement, while at the same time maintaining sufficient yield strength in the central ladder portion.
Accordingly, the present invention has, as one of its objects, the provision of a process for facilitating the making of high strength aluminum ladder rungs through the use of selective heat treatment, as applied to only the ends of the aluminum ladder rungs.
It is additionally an object of the present invention to produce an aluminum ladder rung which has better ductility in its end portions such that during the upsetting of the ends, in order to secure the ladder around and to the end rails or stiles, there is less chance for the metal to crack or fracture.
It is a further object of this invention to produce a ladder rung which has a greater yield strength at its central portion so as to bear greater loads during use--without increasing the rung or stile wall width, overall component size or heft.
It is also an object of the present invention to produce a ladder rung which has less deformation at its central portion of the rung while bearing said heavy loads.
An additional object of the present invention is to enable the manufacture of heavy duty ladders with more consistent desireable results, resulting in less fabrication scrap, time and labor lost, often required by the rigorous standards these ladders must meet.
These and other objects of the invention will become apparent in light of the present specification, drawings and claims.